This invention relates generally to receivers of electrical signals in apparatus such as radar and sonar wherein electromagnetic or acoustic waves are used to "probe" an environment of uncertain composition and the resulting scattered waves from the environment are analyzed for the purpose of determining the nature of the environment. More particularly, the invention relates to the processing methods practiced by the signal processors in such receivers.
The use of probing signals for the identification of objects, inhomogeneities, or disturbances in a wave-propagation-supporting medium has been a basic environmental investigative technique for centuries. However, only since the middle of the twentieth century has the knowledge of signal detection principles and the capability of implementing complicated signal processing designs in hardware resulted in sophisticated signal processors that test the theoretical limits to signal detection and analysis.
The genesis of radar and sonar signal processors were the simple but effective square-law detector and its close relative, the envelope detector, that were used in the detection of radar and sonar pulses in the World War II era. It was subsequently discovered that the so-called correlation processor, which computed the correlation of received signals with replicas of the transmitted signal, was theoretically more effective in extracting the returning signals from the everpresent background noise and interference. Interestingly, the envelope detector can be a very good approximation to the correlation processor when the transmitted signal is a simple pulse and the parameters of the envelope detector are properly chosen.
The square-law detector, the envelope detector, and the correlation detector all share one attribute--they all calculate a quantity proportional to the energy of the received signal. In the case of received signals that are replicas of the transmitted signal, obtained, for example, by reflection from a plane surface, the use of an energy measure as the means of detection is well-supported by theory which shows that with such measures, the highest possible signal-to-noise ratio is obtained. However, there is no reason to believe that energy-measure processors are the most effective detectors of received signals that result from reflection or scattering of the transmitted wave by more complicated media structures and compositions. In such situations one is looking for changes in received signals as a function of time and direction of arrival and not whether a delayed and attenuated version of the transmitted signal is being received. In these circumstances, certain experimental results suggest that entropy measures are significantly more effective than energy measures as a means of detecting such complicated returning signals.